1. Field of the Invention
This invention relates to an apparatus for winding at least one elongate element onto a reel or bobbin so that the element has a constant linear speed. An elongate element can be regarded as one whose length is at least 10 times its cross-section and may comprise e.g. sheets, plastic foil, webs, cables, strands, wires, threads etc.
Reference is made herein to "winding-up apparatus" but this term is intended to encompass also apparatus which operate to unwind an elongate element from a reel or bobbin.
2. Related Art
It is often necessary or desirable to wind an elongate element with an appropriate tension which is as constant as possible. Wire, for example, wound onto a bobbin or reel under a tension which is too high can be damaged, whilst wire wound onto a bobbin or reel with a tension that is too low can cause problems when the wire is subsequently unwound.
In the British Patent Specification No. GB 1,205,162 it has already been shown that if the tension in an elongate element, which is fed to a reel or bobbin at a constant linear speed, is to be kept constant then the effective take-up torque M.sub.E exerted by the reel or bobbin must vary as a hyperbolic function in relation to its rotational speed. The required torque M.sub.M of the driving motor of the reel or bobbin is the sum of the effective take-up torque M.sub.E and of the friction torque M.sub.F : EQU M.sub.M =M.sub.E +M.sub.F
The value of the friction torque M.sub.F is smaller than the value of the effective torque M.sub.E, i.e. M.sub.F is 10 to 50 per cent of M.sub.E. This means that in order to obtain a constant tension in the elongate element the torque/rotational speed characteristic of the motor driving the reel must be approximate to M.sub.M.
A winding apparatus generally comprises a feeding mechanism and a winding bobbin or reel. The feeding mechanism may comprise a motor and a capstan for feeding the elongate element at a constant linear speed.
The prior art provides several proposals for maintaining tension in the elongate element as constant as possible.
In one known winding apparatus, the tension in the elongate element is monitored by means of a "dancer" or feeler lever. If the tension varies, the dancer is pivoted and causes an appropriate correction to be applied to a mechanical or electrical speed control device. This is usually a potentiometer system adapted to change the speed of a DC-motor which drives the bobbin or reel. The "dancer system" gives an actual torque/rotational speed curve that is very close to the desired torque/rotational speed characteristic. The dancer system is, however, costly to install, and also expensive to maintain because of the DC-motor.
The above mentioned British Patent Specification No. GB 1,205,162 discloses a system intended to avoid this draw-back. The winding apparatus has an electrical drive for the reel or bobbin in the form of a capacitor motor having a capacitor of such capacity that the torque/rotational speed characteristic is approximately equal to the desired torque/rotational speed characteristic. This avoids the need for a costly closed-loop control system and the difficulties in the maintenance of DC-motors. However, the bobbin or reel is driven by the motor through a transmission having a constant gear ratio and therefore, if an elongate element has to be wound under a higher tension than a previous element, as is the case e.g. with a wire having a larger diameter than the previous wire, then the transmission is to be changed, i.e. geared "down", so that the reel or bobbin runs more slowly and more torque us applied. This gearing down (or up), i.e. adaptation of the fixed gear ratio, is necessary not only when another winding tension is required, but also when another linear delivery speed of the elongate element is required or when the dimensions of the bobbins or reels are changed. The adaptation of the fixed gear ratio is done mechanically and only when the winding apparatus is not in operation, e.g. by changing the gear ratio of a gear system or of a belt system. This operation is time-consuming and the arrangement cannot always provide the precise gear ratio required for the desired torque and linear speed concerned.